Add support for an array in imports

[analogrelay]

The imports syntax in project.json should support an array so that we can have multiple items. For example:

{
  "frameworks": {
    "netstandard1.2": {
      "imports": [ "dotnet5.3", "portable-net45+win8" ]
    }
  }
}

Also, currently imports must be a portable profile (i.e. portable-...). This restriction should be loosened. At a minimum, dotnet and netstandard (i.e. the new portable TFMs) should be allowed as well. Personally, I think allowing any framework to appear in this list is reasonable, since it is designed as a "Go away NuGet, I know what I want" feature for advanced usage. If someone wants to import net45 into win8, I say let 'em ;)

/cc @yishaigalatzer @lodejard @emgarten @ericstj @davidfowl

[yishaigalatzer]

should this support an Array or a single property? (I'd say yes)

[analogrelay]

Yes, both is best. In general with array properties we allow a string as a short-hand for an array with one element.

[ericstj]

Additionally it looks like the imports is limited to just "PCL" frameworks. That should be opened up to more frameworks.

[analogrelay]

Ah, good point. I'll update the issue to clarify.